the 1x1px offset was really bothering me, so I tracked down the code and this works:
Battlescape/MiniMapView.cpp, ~line 150+
drawLine(centerX - CELL_WIDTH -1, centerY - CELL_HEIGHT -1,
centerX - xOffset -1, centerY - yOffset -1,
color); https:// top left
drawLine(centerX + xOffset -1, centerY - yOffset -1,
centerX + CELL_WIDTH -1, centerY - CELL_HEIGHT -1,
color); https:// top right
drawLine(centerX - CELL_WIDTH -1, centerY + CELL_HEIGHT -1,
centerX - xOffset -1, centerY + yOffset -1,
color); https:// bottom left
drawLine(centerX + CELL_WIDTH -1, centerY + CELL_HEIGHT -1,
centerX + xOffset -1, centerY + yOffset -1,
color); https:// bottom right
If someone with git-fu would like to toss it into a nightly build Pls do. All those "-1"s are yours truly,